1. Field of the Invention
The present invention relates to a method for preventing accumulation of sludge made of powdered coal on a bottom of a coal water mixture (CWM) storage tank.
2. Description of Related Art
When CMW is stored in a storage tank, coal particles settle due to a difference in specific gravity and finally make a layer of sludge on a bottom of the storage tank. In this specification, CMW means a coal water mixture which is liquid made by mixing powdered coal and water.
As the settlement of the sludge advances with passage of time, component ratios become ununiform dependent on a liquid depth, and thereby the sludge is accumulates on the tank bottom. When the settled and accumulated sludge becomes lumpy, not only do the pipes of piping get clogged but also the effective storage capacity of the storage tank is reduced as the lumping occurs on full area and the lumps accumulate. Further, it is necessary to carry out a large scale cleaning operation when inspecting the tank by opening it, which results unfavorable effects including a very big rise in maintenance costs.
There is an example where in a CWM storage tank sludge accumulated 1-2 meters deep for one year of storage period.
The following shows representative physical features of CWM:
______________________________________ Components: Water about 30 wt.% Coal (powdered coal) about 70 wt.% Additives about 0.5 wt.% Coal particle size: .ltoreq. 500 .mu.m 99% .ltoreq. 150 .mu.m 94% Apparent viscosity: 900 .+-. 300 CP on condition that a temperature is 25.degree. C. and a shearing speed is 100 per sec. Specific gravity: 1.25 Calorific value: 5 000 kcal/kg ______________________________________
An example, of a prior art apparatus for preventing accumulation of sludge is disclosed in a Japanese Unexamined Utility Model Publication No. 1-100795. FIG. 8 shows a constitution of this apparatus.
As shown in FIG. 8, referential number 1 denotes a storage tank, 2 a stored liquid, 3 a side wall, 4 a bottom, 7 a liquid suction pipe, 9 a valve, 10 a rotary joint, 11 a circulator pump and 12 a liquid feed pipe. Further, referential number 19 denotes a descending pipe, 21, 22, 23, 24 jet pipes constituting nozzles, 31 a liquid feed branch pipe, 32 a rail, and 33 a travel device. Symbol mark "S" denotes sludge.
The rail 82 is circularly placed on an inside of a cover roof of the storage tank 1, and the travel device 33 equipped with a drive source is hung, being engaged with the rail. The liquid feed branch pipe 31 is horizontally extended near the roof cover and the two descending pipes 19 vertically are branched from the branch pipe 31, provided with the nozzles 21, 22, 23, 24 on lower ends of the descending pipes. The branch pipe 31 and the descending pipes are held by the rail 32.
When the travel device 33 travels during the stored liquid 2 being sent by the circulation pump 11 of the prior art apparatus thus constituted, the descending pipe 19 rotate around a center axis of the storage tank t while the stored liquid 2 is being jetted out of the right and left nozzles 21, 22, 23, 24. Then, sludge "S" which is inclined to settle on the bottom 4 of the storage tank 1 is stirred by the stored liquid 2 jetted out of the nozzles 21, 22, 23, 24 and the accumulation of the sludge is prevented.
In case of the prior art apparatus shown in FIG. 8, however, special consideration was not given to technical study of the flow of the stored liquid jetted out of the nozzles. Consequently, stirring produced by the jet flow of the stored liquid was not sufficiently operated, and there was a disadvantage in that the apparatus was ineffective in preventing accumulation of sludge since it was unable to effectively utilize the jet flow.